There's multiple chicago meetups tho, so we might be talking about separate ones.
This one:
http://www.meetup.com/The-Chicago-Bitcoin-Meetup/It was organized by a guy that went by the name of Igor prior to November.
Over 600 members and meetings with a decent turnout somewhat regularly.
If you're aware of another one, how is the turnout, and how often do they meet? I'd be interested in meeting with a new group of people.